Unstable flow conditions within a turbo compressor can arise from any number of changing process conditions. When this occurs and reduces the flow of gasses with an increase in the specific mechanical energy (polytrophic) of the gas stream, the turbo compressor may surge. Surging can significantly damage a turbo compressor and therefore, control systems have been developed to monitor a turbo compressor's performance.
Should the performance levels drop to a potential surge situation, the control systems must open an anti-surge valve to recycle or blow off an additional portion of the gas flow. If recycling occurs too extensively, it will have an adverse impact on the overall efficiency of the turbo compressor. If recycling is not properly controlled, there may be inadequate protection against surge and the potential damage it may cause to the turbo compressor. There is therefore a need to effectively and efficiently monitor the turbo compressor's operating conditions and evaluate their proximity to the surge conditions. The allowable proximity is commonly known as the safety margin with surge producing operational parameters commonly known as the turbo compressor's surge limit.
A compressor's surge limit, displayed in coordinates of the reduced flow rate (Qr) and the reduced head (Hr) is often very difficult to accurately characterize. This difficulty arises from the quality of current compressor flow measurement methods. The level of difficulty increases further when multistage compressors and compressors with side streams are employed. There is therefore a need to develop a method of controlling the compressor to prevent the compressor from reaching its surge limit that does not rely upon measurements of a reduced flow rate or correspondingly, measurements of the compressor's power.
There is therefore a need for a method of controlling a turbo compressor to avoid surge limit conditions which avoids these and other problems.